Container insertion and removal

[Robert Jacques]

On Sun, 07 Mar 2010 22:07:14 -0500, Steven Schveighoffer  
<schveiguy at yahoo.com> wrote:
> On Sun, 07 Mar 2010 12:43:09 -0500, Robert Jacques <sandford at jhu.edu>  
> wrote:
>> On Sun, 07 Mar 2010 08:23:03 -0500, Steven Schveighoffer  
>> <schveiguy at yahoo.com> wrote:
[snip]
>> Please define for me an O(1) slice or index operation for a linked-list.
>
> One for which you have references to the slice end points.  I think this  
> will work, and I was planning on providing it in the upcoming  
> dcollections port.  The only thing you cannot guarantee is that the  
> order is correct.

The container would have to do an O(N) search to verify the ranges are  
actually part of the collection. And using two ranges as iterators to  
create a third range feels very wrong and very bug prone: see all the  
issues raised during Andrei's iterators vs ranges presentations.  
Similarly, it feels wrong for something to define slicing and not indexing.

>> By the way, having ranges detect if they reach their end nodes or not  
>> is fairly easy to do.
>
> you are correct in that point, you could throw an exception as long as  
> the end point is part of the range structure.  If you just use a current  
> node plus a length, then you cannot do that.  But soft ops are not  
> necessary to create this.

Soft ops are necessary to document in code whether that invalidation is  
expected to happen.

>>
>>> I still fail to see the difference between "soft" operations and  
>>> non-soft.  What does soft guarantee?  Give me a concrete definition,  
>>> an example would help too.
>>
>> There are a couple of possible definitions for soft operations: 1) the  
>> memory safety of the ranges of a collection are guaranteed. 2) That for  
>> the topology viewed by a range isn't logically changed. i.e. the range  
>> will continue to perform the same logical function if the topology its  
>> operating on is updated 3) That for the topology viewed by a range  
>> isn't actually changed and all elements selected at range creation will  
>> be viewed. 4) Like 3, but with all values being viewed.
>>
>> For example, modifying an array in any way doesn't change 1, 2 or 3 for  
>> any of its slices.
>> For a linked list defining a forward range, mutation, insertion and  
>> removal can be done under 1 & 2.
>> The same can be said about doubly linked lists and bidirectional ranges.
>> For other containers, such as a sorted tree, mutation can break a 2/3  
>> though insertion and deletion don't break 2.
>> Although, the ranges will see many values, they may not see all the  
>> values currently in the collection nor all the values in the collection  
>> when the iterator was generated. So code that relies on such properties  
>> would be logically invalid.
>>
>> I'd probably define hard ops as being 1) and soft ops at level 2. 4) is  
>> really only possible with immutable containers.
>
> Hard ops definitely qualify as #1, since we are in a GC'd language.
>
> I don't really understand 2, "the range will continue to perform the  
> same logical function," what does that mean?  Define that function.  I  
> would define it as #3, so obviously you think it's something else.
>
> #3 would be most useful for soft operations if it could be guaranteed.   
> I don't think it can.

I was thinking of "iterate from the start to the end", for example. One  
might better describe this concept as the topology of the container  
relative to the range doesn't change: things before it stay before, things  
after it stay after and in the case of bidirectional ranges, things in the  
middle, stay in the middle.

>>>> Wouldn't re-hashing necessitate re-allocation? (Thus the range would  
>>>> see a
>>>> stale view)
>>>
>>> God no.  If my hash collision solution is linked-list based (which it  
>>> is in dcollections), why should I reallocate all those nodes?  I just  
>>> rearrange them in a new bucket array.
>>
>> Sorry, I was assuming that if you were going to implement a hash  
>> collection you wouldn't be using a linked list approach, since that's  
>> what D's associative arrays already do. The are some really good  
>> reasons to not use a list based hash in D due to GC false pointer  
>> issues, but basically none to re-implementing (poorly?) D's built-in  
>> data structure.
>
> Hm... my hash outperforms builtin AAs by a wide margin.  But this is not  
> technically because my implementation is better, it's because AA's use  
> "dumb" allocation methods.  I don't know about false pointers, the hash  
> nodes in my implementation only contain pointers, so I'm not sure there  
> is any possibility for false ones.

The GC isn't precise, so if you have a non-pointer type in a structure  
with a pointer or in a class, you'll get false pointers. (i.e. the hash  
value at each node)

>>>> The difference is that algorithms can document in their template
>>>> constraints that they need a container with 'soft' properties.
>>>
>>> What is the advantage?  Why would an algorithm require soft  
>>> functions?  What is an example of such an algorithm?
>>
>> Something that uses toUpperCase or toLowerCase, for example.
>
> I guess I won't get a real example.  I'm not sure it matters.  When  
> Andrei starts implementing the soft methods, either they will be a huge  
> win or obviously useless.  If I were a betting man, I'd bet on the  
> latter, but I'm not really good at betting, and Andrei's ideas are  
> usually good :)

Though this was the first thing that popped into my head, it is a fairly  
real example. Consider you are doing some processing involving a container  
and you call a library function, like toUpperCase, that will perform some  
mutation. For some containers, this is completely reasonable. But for  
others, the container topology is going to massively change, invalidating  
all your current ranges. And you really like to guarantee that both the  
current implementation and the one 6 months from now don't mess up your  
ranges and cause a bunch of exceptions to be thrown.

>>>> I wasn't thinking of multi-threaded containers. I was trying to point  
>>>> out
>>>> that version ids have failed in lock-free containers, where things are
>>>> happening on the order of a single atomic op or a context switch.  
>>>> Given
>>>> the time a range could go unused in standard code, versioning won't  
>>>> work.
>>>
>>> Are you trying to say that if you don't use your range for exactly  
>>> 2^32 mutations, it could mistakenly think the range is still valid?   
>>> That's a valid, but very very weak point.
>>
>> Umm, no. That is a valid point that happens in production code to  
>> disastrous effects. Worse, it doesn't happen often and there's no good  
>> unit test for it. I for one, never want to debug a program that only  
>> glitches after days of intensive use in a live environment with real  
>> customer data. Integer overflow bugs like this are actually one of the  
>> few bugs that have ever killed anyone.
>
> I think you are overestimating the life of ranges on a container.  They  
> will not survive that many mutations, and the worst that happens is the  
> range continues on fully allocated memory (i.e. your #1 above).

A year ago I would have agreed with you, but then I saw a couple of  
articles about how this unlikely event does start to occur repeatably in  
certain circumstances. This made a bunch of news since it threw a massive  
spanner in lock-free containers, which relied on this never happening.

> You can also force the container to invalidate itself once the first  
> wrap occurs.  This at least will be a hard error.

So every container will have a finite number of operation and that's it?

>>>> No it's not. version tags + integer overflow = bug. Doug Lea knew  
>>>> about
>>>> the problem and but thought it would never happen in real code. And  
>>>> Bill
>>>> Gates thought no one will need more than 640k of ram. They both have  
>>>> been
>>>> proven wrong.
>>>
>>> Overflow != bug.  Wrapping completely to the same value == bug, but is  
>>> so unlikely, it's worth the possibility.
>>
>> Statistics 101: do a test enough times and even the highly improbable  
>> will happen.
>
> In statistics we generally ignore the outliers.  I normally am on your  
> side in these types of cases, but we are talking about a statistical  
> impossibility -- nobody will leave a range untouched for exactly 2^32  
> mutations, it simply won't happen.  Your computer will probably be  
> obsolete before it does.

No, this is a statistical implausibility. One that has already happened to  
some people. ulongs on the other hand...

> BTW, I'm not advocating adding mutation counters, I don't plan on adding  
> them.  It's just another alternative to "soft" mutations.

I understand.